Dichroic prism, prism unit, and projection display apparatus

ABSTRACT

At least one of four rectangular prisms in a dichroic prism is made longer than the other rectangular prisms. Parts of the rectangular surfaces of the long rectangular prisms partially protrude from the rectangular surfaces of the other rectangular prisms in the longitudinal direction. The protruding parts of the rectangular surfaces of the long rectangular prisms are not provided with a dichroic film. Such a construction makes it possible to prevent return light, which returns from the light emitting surface side of the dichroic prism and the dichroic prism, from emitting again from the light emitting surface.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a dichroic prism, a prism unit thereof,and a projection display apparatus.

2. Description of Related Art

Most projection display apparatuses for projecting a color image onto aprojection screen utilize a dichroic prism. A dichroic prism is anoptical element that synthesizes and emits lights of three colors, red,green, and blue, in the same direction.

FIG. 13 is a conceptual view showing the principal part of a projectiondisplay apparatus. This projection display apparatus includes threeliquid crystal light valves 42, 44, and 46, a dichroic prism 48, and aprojection lens 50. Red reflecting films 48R and blue reflecting films48B are located in the form of a cross in the center of the dichroicprism 48. The dichroic prism 48 synthesizes lights of three colors, red,green, and blue that are modulated by the three liquid crystal lightvalves 42, 44, and 46, and emits the lights toward the projection lens50. The projection lens 50 focuses the synthesized lights onto aprojection screen 52.

A general type of dichroic prism is made by bonding rectangular surfacesof four rectangular prisms that are equal in size. The red reflectingfilms 48R are previously formed on predetermined rectangular surfaces oftwo rectangular prisms so that they form a flat plane when fourrectangular prisms are bonded together. The blue reflecting films 48Bare also previously formed on predetermined rectangular surfaces of tworectangular prisms in a similar manner. In making a dichroic prism bybonding four rectangular prisms of equal size, however, it is verydifficult to accurately bond the rectangular prisms so that the redreflecting films 48R form a flat plane and the blue reflecting films 48Bform a flat plane.

Accordingly, in order to accurately bond rectangular prisms together, itis well known to make some of the rectangular prisms longer than theothers, as shown in, for example, FIG. 1 of Japanese Unexamined PatentPublication No. 7-294845.

FIGS. 14(A) and 14(B) are explanatory views showing problems of such aconventional dichroic prism. As shown in FIG. 14(A), a dichroic prism 48includes two rectangular prisms 61 and 62 that are long in thelongitudinal direction (also referred to as "a long rectangular prismpair"), and two rectangular prisms 63 and 64 that are short in thelongitudinal direction (also referred to as "a short rectangular prismpair"). Blue reflecting films 48B are formed on interface surfacesbetween the long rectangular prism pair 61 and 62 and the shortrectangular prism pair 63 and 64. Rectangular surfaces of the longrectangular prism pair 61 and 62 are partly exposed, and the bluereflecting films 48B are also formed on the exposed surfaces. Moreover,red reflecting films are formed on an interface surface between the longrectangular prism pair 61 and 62, and on an interface surface betweenthe short rectangular prism pair 63 and 64, respectively.

In a projection display apparatus, there is light that returns from aprojection lens 50 toward the dichroic prism 48 because of reflection bythe projection lens 50, or the like. A description will now be given ofa case in which white return light W is produced in the example shown inFIGS. 14(A) and 14(B). FIG. 14(B) is a horizontal sectional view ofprotruding portions of the long rectangular prism pair 61 and 62 at thetop thereof. When the return light W enters a protruding rectangularsurface of the rectangular prism 61 (a surface provided with the bluereflecting film 48B), it is totally reflected by the rectangularsurface. Only red light R of the totally reflected return light W isreflected by the red reflecting film 48R, and is emitted again towardthe projection lens 50.

In the conventional dichroic prism shown in FIGS. 14(A) and 14(B), thereturn light, which returns from the light emitting surface side to thedichroic prism, is thus reflected inside the dichroic prism and isemitted again from the light emitting surface. Consequently, theinfluence of this return light is exerted on an image to be reproducedby light emitted from the dichroic prism.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above problem of therelated art, and it is an object of the present invention to preventreturn light, which returns from the light emitting surface side of adichroic prism to the dichroic prism, from emitting again from the lightemitting surface.

In order to at least partially solve the above-mentioned problem, afirst embodiment provides a dichroic prism composed of four rectangularprisms with their rectangular surfaces bonded together, wherein at leastone of the four rectangular prisms is longer than the other rectangularprisms, so that a part of the rectangular surface of the longrectangular prism protrudes from the rectangular surfaces of the otherrectangular prisms in the longitudinal direction, and wherein a dichroicfilm is formed on a part of the rectangular surface of the longrectangular prism other than the protruding part.

Since this makes it possible to prevent return light from beingreflected by a dichroic film on the protruding part, it is possible toprevent return light from emitting again from the light emittingsurface.

A second embodiment provides a dichroic prism composed of fourrectangular prisms with their rectangular surfaces bonded together,wherein a first rectangular prism pair composed of adjoining two of thefour rectangular prisms is longer than the other second rectangularprism pair, so that a part of the rectangular surface of the firstrectangular prism pair protrudes from the rectangular surface of thesecond rectangular prism pair in the longitudinal direction, and whereina dichroic film is formed on a part of the rectangular surface of thefirst rectangular prism pair other than the protruding part.

Since the second embodiment also makes it possible to prevent returnlight from being reflected by a dichroic film on the protruding part, itis possible to prevent return light from emitting again from the lightemitting surface.

In the above first or second embodiment, it is preferable that the tworectangular prisms in the first rectangular prism pair be fixed in astate shifted from each other in the longitudinal direction so that theyform a step.

This makes it possible to precisely position the center axis of thedichroic prism with the use of the step, and furthermore, to preciselyposition the reflecting surfaces of the rectangular prisms in the sameplane.

In the above first or second embodiment, it is preferable that therectangular surface at the step of the first rectangular prism pair beprovided with a light diffusing layer for diffusing light. This makes itpossible to prevent return light from being totally reflected by thestep of the protruding part.

The light diffusing layer may be an adhesive layer or a ground glasslayer. This ground glass layer may be formed by omitting to grind a partof the rectangular surface of the rectangular prism that is to beprovided with the light diffusing layer.

A third embodiment provides a prism unit comprising a dichroic prism ofthe first or second embodiment, and a prism stand for mounting thedichroic prism thereon, wherein the prism stand has a step that matchesthe step of the dichroic prism.

The use of such a prism unit makes it possible to easily mount thedichroic prism of the first or second embodiment in another apparatus.

A fourth embodiment provides a projection display apparatus comprisingan illumination optical system for emitting illumination light, coloredlight separation means for separating the illumination light into lightsof three colors, three light modulation means for modulating the threecolored lights based on a given image signal, a dichroic prism of thefirst or second embodiment, and a projection optical system forprojecting the lights synthesized by the dichroic prism.

In the projection display apparatus of the fourth embodiment, returnlight that returns from the projection optical system to the dichroicprism can be prevented from being reflected inside the dichroic prismand emitting again from the light emitting surface. As a result, it ispossible to prevent an image to be reproduced by light projected fromthe projection optical system from being affected by the return light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A) and 1(B) are explanatory views of a dichroic prism 200according to a first embodiment of the present invention.

FIGS. 2(A) and 2(B) are a front view of the dichroic prism 200 of thefirst embodiment, and a cross-sectional view taken along line B--B inthe front view.

FIGS. 3(A) to 3(D) are perspective views of four rectangular prisms 301to 304 that constitute the dichroic prism 200 of the first embodiment.

FIG. 4 is a perspective view of a dichroic prism 200a according to asecond embodiment of the present invention.

FIGS. 5(A) and 5(B) are a front view of the dichroic prism 200a of thesecond embodiment, and a cross-sectional view taken along line B--B inthe front view.

FIGS. 6(A) to 6(D) are perspective views of four rectangular prisms301a, 302a, 303, and 304 that constitute the dichroic prism 200a of thesecond embodiment.

FIG. 7 is an explanatory view showing a process of bonding the firstrectangular prism 301a and the third rectangular prism 303.

FIG. 8 is an explanatory view showing a process of bonding the secondrectangular prism 302a and the fourth rectangular prism 304.

FIGS. 9(A) and 9(B) are explanatory views showing a method of assemblingtwo bonded prisms made according to the processes shown in FIGS. 7 and8.

FIGS. 10(A) and 10(B) are perspective views of a prism stand 400 used inthe dichroic prism.

FIGS. 11(A) to 11(C) are explanatory views of a prism unit 260 employedin a projection display apparatus.

FIG. 12 is a schematic plan view showing the principal part of theprojection display apparatus that utilizes the dichroic prism unit 260according to the embodiment of the present invention.

FIG. 13 is a conceptual view showing the principal part of a projectiondisplay apparatus.

FIGS. 14(A) and 14(B) are explanatory views showing a problem of aconventional dichroic prism.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A. First Embodiment:

Next, modes for carrying out the present invention will be describedaccording to the embodiments. FIGS. 1(A) and 1(B) are explanatory viewsof a dichroic prism 200 according to a first embodiment of the presentinvention. As shown in FIG. 1(A), this dichroic prism 200 includes twoadjoining long rectangular prisms 301 and 302 (also referred to as "along rectangular prism pair"), and two adjoining short rectangularprisms 303 and 304 (also referred to as "a short rectangular prismpair"). The two long rectangular prisms 301 and 302 are shaped likerectangular triangular prisms that are equal in length (in thelongitudinal direction). The two short rectangular prisms 303 and 304are also shaped like rectangular triangular prisms that are equal inlength. The short rectangular prism pair 303 and 304 are bonded toalmost the center of the long rectangular prism pair 301 and 303 in thelongitudinal direction. That is, the long rectangular prism pair 301 and302 protrude from the top and bottom of the short rectangular prism pair301 and 302 by an almost equal length.

FIG. 2(A) is a front view of the dichroic prism 200 according to thefirst embodiment, and FIG. 2(B) is a cross-sectional view taken alongline B--B in FIG. 2(A). As shown in FIG. 2(B), blue reflecting films301B and 302B are formed on an interface surface between the longrectangular prism pair 301 and 302 and the short rectangular prism pair303 and 304. Furthermore, red reflecting films 302R and 304R are formedon an interface surface between the long rectangular prism pair 301 and302 and on an interface surface between the short rectangular prism pair303 and 304, respectively.

FIGS. 3(A) to 3(D) are perspective views of the four rectangular prisms301 to 304 that constitute the dichroic prism 200 of the firstembodiment. As shown in FIG. 3(A), the blue reflecting film 301B isformed on one of two rectangular surfaces of the first rectangular prism301 that serves as an interface surface between the first rectangularprism 301 and the third rectangular prism 303, providing that exposedsurfaces 311 and 312 protruding from the short rectangular prism pair303 and 304 are not provided with the blue reflecting film 301B, and areground to flat ground surfaces. The other rectangular surface of thefirst rectangular prism 301 is also not provided with any reflectingfilm, and is ground to a flat ground surface.

As shown in FIG. 3(B), the blue reflecting film 302B is formed on one oftwo rectangular surfaces of the second rectangular prism 302 that servesas an interface surface between the second rectangular prism 302 and thefourth rectangular prism 304, providing that exposed surfaces 321 and322 protruding from the short rectangular prism pair 303 and 304 are notprovided with the blue reflecting film 301B and are ground to flatground surfaces. The other rectangular surface of the second rectangularprism 302 (i.e., the surface serving as an interface surface between thesecond rectangular prism 302 and the first rectangular prism 301) isprovided with the red reflecting film 302R, providing that surfaceportions 331 and 332 protruding from the short rectangular prism pair303 and 304 are not provided with the red reflecting film 302R, and areground to flat ground surfaces.

The two rectangular surfaces of the third rectangular prism 303 (FIG.3(C)) are not provided with reflecting films (dichroic films), and areground to flat ground surfaces. One of the two rectangular surfaces ofthe fourth rectangular prism 304 (FIG. 3(D)), which serves as aninterface surface between the fourth rectangular prism 304 and the thirdrectangular prism 303, is entirely provided with the red reflecting film304R. The other rectangular surface of the fourth rectangular prism 304is not provided with any reflecting film (dichroic film), and is groundto a flat ground surface.

As shown in FIGS. 3(A) and 3(B), the reflecting films 301B, 302B, and302R are formed only on the parts of the rectangular surfaces of thelong rectangular prism pair 301 and 302 that overlap with the shortrectangular prism pair 303 and 304. In other words, the portions 311,312, 321, 322, 331, and 332 protruding from the top and bottom of theshort rectangular prism pair 303 and 304 are not provided with anyreflecting films (dichroic films).

The reflecting film (dichroic film) is generally formed by evaporating adielectric multilayer film. The dichroic prism 200 shown in FIG. 1 canbe produced by bonding the thus prepared four rectangular prisms 301 to304 with an adhesive. A method of assembling the dichroic prism of theembodiment will be described later in detail.

FIGS. 1(A) and 1(B) also show the optical path of white return light Wthat enters the protruding portions of the long rectangular prism pair301 and 302. FIG. 1(B) is a horizontal sectional view of the protrudingportions of the long rectangular prism pair 301 and 302. When the returnlight W from a projection lens 270 is incident on the protrudingrectangular surface of the second rectangular prism 302 (the exposedsurface 321), it is totally reflected thereby. Since the interfacesurfaces between the protruding portions of the long rectangular prismpair 301 and 302 (the surfaces 331 and 332 shown in FIG. 3(B)) are notprovided with a red reflecting film, the return light W totallyreflected by the exposed surface 321 shown in FIG. 1(A) travels straightand unchanged through the interface surface between the long rectangularprism pair 301 and 302, passes through the first rectangular prism 301,and then, emerges.

In this way, in the dichroic prism 200 of the first embodiment, thereturn light W from the projection lens 270 does not emit again from thelight emitting surface of the dichroic prism 200 (i.e., the lightemitting surface of the second rectangular prism 302). As a result, itis possible to avoid a phenomenon in which return light affects an imageto be reproduced by light emitted from the light emitting surface of thedichroic prism. This advantage is obtained because dichroic films arenot formed on the interface surfaces 331 and 332 between the protrudingportions.

Furthermore, since the exposed surfaces 311, 321, 312, and 322 are notprovided with a dichroic film in the above-described embodiment,assembly precision can be improved by assembling the dichroic prism 200with reference to the exposed surfaces.

B. Second Embodiment

FIG. 4 is a perspective view of a dichroic prism 200a according to asecond embodiment of the present invention. The dichroic prism 200a isdifferent from the first embodiment shown in FIGS. 1(A) and 1(B) in thatlong rectangular prisms 301a and 302a are vertically shifted from eachother by a predetermined amount.

FIG. 5(A) is a front view of the dichroic prism 200a of the secondembodiment having a width WP, and FIG. 5(B) is a cross-sectional viewtaken along line B--B of FIG. 5(A). An upward protrusion length H1 ofthe first rectangular prism 301a (i.e., the height of an exposed surface341) is equal to the downward protrusion length of the secondrectangular prism 302a (i.e., the height of an exposed surface 352).Furthermore, a downward protrusion length H2 of the first rectangularprism 301a (the height of an exposed surface 342) is equal to the upwardprotrusion length of the second rectangular prism 302a (the height of anexposed surface 351). When the long rectangular prisms 301a and 302a arethus shifted from each other in the longitudinal direction, it is easyto precisely position the center axis of the dichroic prism 200a inmounting the dichroic prism 200a in an optical apparatus such as aprojection display apparatus. In addition, it is easy to position thereflecting surfaces of dichroic films in the same plane.

FIGS. 6(A) to 6(D) are perspective views of four rectangular prisms301a, 302a, 303, and 304 that constitute the dichroic prism 200a of thesecond embodiment. The long rectangular prism pair 301a and 302a shownin FIGS. 6(A) and 6(B) are different from those shown in FIGS. 3(A) and3(B) only in the positions where reflecting films (dichroic films) areformed.

FIG. 7 is an explanatory view showing the process of combining the firstrectangular prism 301a and the third rectangular prism 303. Thecombination is performed by using a first assembly jig 500. The firstassembly jig 500 includes a base 502 having a flat surface 502a, and aheight difference setting member 504 fixed on the base 502. In theheight difference setting member 504, a height difference H1 thatcorresponds to a height difference H1 between the two rectangular prisms301a and 303 is formed with high accuracy.

In combining the two rectangular prisms 301a and 303, first, an adhesiveis applied onto their surfaces to be bonded. Then, air bubbles in theadhesive are removed by rubbing the two prisms together. After that, asshown in FIG. 7, the two prisms 301a and 303 are laid on the base 502.At this time, the prisms 301a and 303 are pressed against the heightdifference setting member 504 so that the height difference between theprisms is equal to the height difference H1 of the height differencesetting member 504. This makes it possible to set the height differencebetween the prisms with high precision.

The bottom surfaces of the two rectangular prisms 301a and 303 shown inFIG. 7 are not provided with any dichroic films, and are formed withground surfaces. Since the surface 502a of the base 502 is formed with ahigh-precision flat surface, it is possible to obtain a high degree offlatness with respect to one plane formed by the two rectangular prisms.

After the relative positional relationship between the two rectangularprisms 301a and 303 is thus precisely set, the adhesive applied on thebonded surfaces is solidified. As a result, it is possible to obtain therectangular prism pair 301a and 303 in a combination of high precision.

FIG. 8 is an explanatory view showing the process of bonding the secondrectangular prism 302a and the fourth rectangular prism 304. A secondassembly jig 510 to be used at this time includes a base 512 and aheight difference setting member 514 in the same manner as the firstassembly jig 500. The second assembly jig 510 is different from thefirst assembly jig 500 only in that a height difference H2 of the heightdifference setting member 514 is set at a value equivalent to a heightdifference H2 between the two rectangular prisms 302a and 304. Theassembly method is similar to the method for the first rectangular prism301a and the third rectangular prism 303 mentioned with reference toFIG. 7.

The bottom surfaces of the two rectangular prisms 302a and 304 shown inFIG. 8 are provided with red reflecting films, respectively. Since asurface 512a of the base 512 is formed in a high-precision flat surface,it is possible to obtain a high degree of flatness with respect to a redreflecting plane formed by the red reflecting films of the tworectangular prisms 302a and 304.

Comparing lights of two colors, red and blue, to be reflected by thereflecting plane, the red light has the higher spectral luminousefficiency (that is, it is more visible to the naked eye). Therefore, itis preferable that the plane to be formed by the red reflecting films beas even as possible. According to the assembly method of thisembodiment, the assembling can be performed so that the red reflectingfilms 302R and 304R shown in FIGS. 6(B) and 6(D) precisely form oneplane, and therefore, it is possible to produce an excellent dichroicprism in this aspect. Since the spectral luminous efficiencies of green,red, and blue decrease in this order, when green reflecting films areused instead of the red reflecting films or the blue reflecting films,it is preferable that the assembling is performed by this assemblymethod so that the green reflecting films form one plane.

FIGS. 9(A) and 9(B) are explanatory views showing the process of bondingtwo bonded prisms that are produced according to the processes shown inFIGS. 7 and 8. This assembly process uses a third assembly jig 520. FIG.9(B) is a plan view of the third assembly jig 520. The third assemblyjig 520 includes a base 522 having a rectangular opening 522b at aboutthe center, and a height difference setting member 524 fixed on asurface 522a of the base 522. The dimensions of the opening 522b are setso that the short rectangular prism pair 303 and 304 are completely heldinside thereof and both ends of the long rectangular prism pair 301a and302a in the longitudinal direction are outside thereof.

A height difference H3 of the height difference setting member 524 isset at a value equal to the difference between the height difference H1of the bonded prism shown in FIG. 7 and the height difference H2 of thebonded prism shown in FIG. 8 (H2-H1).

In bonding the two bonded prisms together, an adhesive is applied ontotheir surfaces to be bonded, and the two bonded prisms are laid on thethird assembly jig 520 so that the short rectangular prism pair 303 and304 are held inside the opening 522b, as shown in FIG. 9(A). Then, theheight difference between the long rectangular prism pair 301a and 302ais made equal to the height difference H3 of the height differencesetting member 524 by pressing the long rectangular prism pair 301a and302a against the height difference setting member 524. As a result, itis possible to set the height difference between the rectangular prisms301a and 302a with high precision.

Only the protruding portions of the rectangular surfaces of the tworectangular prisms 301a and 302a which are not provided with reflectingfilms (341, 342, 351, and 352 shown in FIGS. 6(A) and 6(B)) are incontact with the surface 522a of the base 522. Since these protrudingportions 341, 342, 351, and 352 are ground surfaces, it is possible toobtain a high degree of flatness with respect to these ground surfacesby placing these protruding portions 341, 342, 351, and 352 on thesurface 522a of the base 522. As shown in FIGS. 6(A) and 6(B), theground surface (adhesive surface) that is flush with the protrudingportions 341 and 342 is provided with the blue reflecting film 301B, andthe ground surface (adhesive surface) that is flush with the protrudingportions 351 and 352 is also provided with the blue reflecting film302B. Therefore, by assembling the rectangular prisms as shown in FIGS.9(A) and 9(B), a high degree of flatness can be obtained with respect tothe blue reflecting plane formed by the blue reflecting films 301B and302B.

After the relative positional relationship between the two bonded prismsis precisely set, the adhesive applied on the bonded surfaces issolidified. As a result, it is possible to obtain the precisely combineddichroic prism 200a (FIG. 4). When an ultraviolet-curing adhesive isused as the adhesive for bonding the four rectangular prisms, it ispossible to reduce the curing time and the heat to be generated duringcuring.

FIG. 10(A) is a perspective view of a prism stand 400 for mounting thedichroic prism 200a thereon, viewed from the front side, and FIG. 10(B)is a perspective view of the prism stand 400, viewed from the oppositeside. This prism stand 400 has an external shape of a square when viewedin plan, and comprises first and second triangular stands 402 and 404having a right-angled triangular shape that are each one-fourth of theexternal shape, and a third triangular stand 406 having a right-angledtriangular shape that is half the external shape. The third triangularstand 406 is the highest of the three triangular stands. The firsttriangular stand 402 is lower than the third triangular stand 406 by H4.This height difference H4 is almost equal to the downward protrusionlength H2 of the first rectangular prism 301a from the short rectangularprism pair 303 and 304, as shown in FIG. 5(A). The second triangularstand 404 is lower than the third triangular stand 406 by H5. Thisheight difference H5 is almost equal to the upward protrusion length H1of the first rectangular prism 301a from the short rectangular prismpair 303 and 304, as shown in FIG. 5(A).

In utilizing the dichroic prism 200a in an optical apparatus such as aprojection display apparatus, preferably, the dichroic prism 200a andthe prism stand 400 are combined into the prism unit shown in FIG. 11(A)by bonding the bottom surface of the dichroic prism 200a to the prismstand 400, and the dichroic prism 200a is fixed inside the opticalapparatus through the prism stand 400. Since the prism stand 400 is flatat the bottom, it can be easily fixed inside the optical apparatus byusing screws or the like. It is preferable to use resin or metal as thematerial of the prism stand in consideration of costs and ease ofshaping. The use of a resin or metal prism stand creates a concern thatthe prism stand is apt to be deformed by heat and the dichroic prism200a in the optical apparatus may be displaced due to the deformation.When the bottom surface of a corresponding rectangular prism of thedichroic prism 200a is bonded to only one of the first to thirdtriangular stands 402, 404, and 406 of the prism stand 400, however, thedichroic prism 200a is relatively resistant to displacement even if theprism stand 400 undergoes thermal deformation. In particular, when therectangular prism 301a is bonded only to the first triangular stand 402,or when the rectangular prism 302a is bonded only to the secondtriangular stand 404, since the area of the bonded surface is small, itis possible to minimize the influence of thermal deformation of theprism stand.

In using the dichroic prism 200 shown in FIGS. 1(A) and 1(B) accordingto the first embodiment, a prism stand having a step portion thatcorresponds to the step portion at the bottom of the dichroic prism 200is used.

FIG. 11(B) is an enlarged view showing the step portion formed by theupward protruding portions of the long rectangular prism pair 301a and302a. A part 310 of the rectangular surface of the second rectangularprism 302a is exposed above the first rectangular prism 301a. Thisexposed surface 310 is provided with an adhesive layer 312 serving as alight scattering layer. FIG. 11(C) is a horizontal sectional view of thedichroic prism 200a, taken at the exposed surface 310 of the secondrectangular prism 302a, and illustrates the state in which return lightW is irregularly reflected by the adhesive layer 312. The return lightW, which returns from the light emitting surface side to the inside ofthe dichroic prism 200a, is totally reflected by the exposed surface 351of the rectangular prism 302a, and then is irregularly reflected againby the adhesive layer 312 formed on another exposed surface 310.Therefore, it is possible to prevent the return light W from emittingagain from the light emitting surface of the dichroic prism 200a.

Another light diffusing layer may be formed on the protruding portion310 instead of the adhesive layer. For example, the exposed surface 310may be made of ground glass. Similarly, it is preferable that theexposed surface 351 and the step portion at the bottom of the dichroicprism be provided with a light diffusing layer such as an adhesivelayer, or be made of ground glass.

C. Configuration of Projection Display Apparatus

FIG. 12 is a schematic plan view showing the principal part of aprojection display apparatus that utilizes the dichroic prism unit 260according to the embodiment of the present invention. This projectiondisplay apparatus comprises an illumination optical system 100, dichroicmirrors 210 and 212, reflecting mirrors 220, 222, and 224, relay lenses230 and 232, three field lenses 240, 242, and 244, three liquid crystallight valves (liquid crystal panels) 250, 252, and 254, the dichroicprism unit 260, and a projection lens system 270.

The illumination optical system 100 comprises a light source 110 foremitting an almost parallel beam, a first lens array 120, a second lensarray 130, a polarizing conversion element 140 for converting incidentlight into a predetermined linearly polarized light component, areflecting mirror 150, and a condenser lens 160. The illuminationoptical system 100 is an optical system for almost uniformlyilluminating the three liquid crystal light valves 250, 252, and 254that are regions to be illuminated.

The light source 110 comprises a source lamp 112 serving as a radiationsource for emitting radial beams and a concave mirror 114 for emittingthe radial light emitted from the source lamp 112 as an almost parallelbeam. It is preferable to use a parabolic mirror as the concave mirror114.

A parallel beam emitted from the light source 110 is split into aplurality of partial beams by the first and second lens arrays 120 and130. Microlenses 122 of the first lens array 120 focus the partial beamsnear polarizing separation films of the polarizing conversion element140. Microlenses 132 of the second lens array 130 have the function offocusing light source images in the first lens array 120 onto the liquidcrystal light valves 250, 252, and 254. The partial beams emitted fromthe microlenses 132 of the second lens array 130 are reflected by thereflecting mirror 150. The condenser lens 160 functions as asuperimposing optical system that superimposes and focuses theseplurality of partial beams onto the liquid crystal light valves 250,252, and 254 serving as regions to be illuminated. As a result, theliquid crystal light valves 250, 252, and 254 are illuminated almostuniformly.

The two dichroic mirrors 210 and 212 function as colored lightseparation means for separating white light condensed by the condenserlens 160 into colored lights of three colors, red, green, and blue. Thefirst dichroic mirror 210 transmits a red light component of the whitebeam emitted from the illumination optical system 100, and reflects ablue light component and a green light component. The red lighttransmitted through the first dichroic mirror 210 is reflected by thereflecting mirror 220, and reaches the liquid crystal light valve forred light 250 through the field lens 240. The field lens 240 has thefunction of focusing the light source image near the second lens array130 into the projection lens system 270. Furthermore, the partial beamspassed through the field lens 240 are made into almost parallel beams.This also applies to the field lenses 242 and 244 located in front ofthe other liquid crystal light valves. The green light of the blue andgreen lights reflected by the first dichroic mirror 210 is reflected bythe second dichroic mirror 212, passes through the field lens 242, andreaches the liquid crystal light valve for green light 252. On the otherhand, the blue light passes through the second dichroic mirror 212 and arelay lens system including the relay lenses 230 and 232 and thereflecting mirrors 222 and 224, further passes through the field lens244, and reaches the liquid crystal light valve for blue light 254. Therelay lens system is provided for the blue light because the opticalpath length of the blue light is longer than those of the other coloredlights.

The three liquid crystal light valves 250, 252, and 254 function asoptical modulation means for forming images by modulating the coloredlights of three colors according to given image information (imagesignals). The dichroic prism unit 260 functions as colored lightsynthesizing means for forming a color image by synthesizing the threecolored lights. In the dichroic prism unit 260, a dielectric multilayerfilm for reflecting red light and a dielectric multilayer film forreflecting blue light are located in about the shape of an X on theinterface surfaces among four rectangular prisms. The three coloredlights are synthesized by these dielectric multilayer films, whereby asynthetic light for projecting a color image is formed. The syntheticlight generated by the dichroic prism unit 260 is emitted toward theprojection lens system 270. The projection lens system 270 functions asa projection optical system for projecting the synthetic light onto aprojection screen 300, and thereby displaying a color image.

This projection display apparatus utilizes the prism unit 260 employingthe dichroic prism according to the above-mentioned embodiment.Therefore, it is possible to prevent return light from the projectionlens 270 to the dichroic prism from emitting again from the lightemitting surface of the dichroic prism. As a result, it is possible toprevent an image to be projected on the projection screen 300 from beingaffected by the return light, and to thereby project a clear image.

The present invention is not limited to the above-mentioned embodimentsand forms, and may be embodied in various forms without departing fromthe spirit and scope thereof. For example, the following modificationsare possible.

(1) While two of the four rectangular prisms for constituting thedichroic prism are longer than the other two rectangular prisms in theabove embodiments, the present invention is not limited to such astructure, and is applicable to a case in which at least one rectangularprism is longer than the other rectangular prisms. Furthermore, thepresent invention is applicable to a case in which four rectangularprisms of equal length are bonded while being in a state shifted fromone another in the longitudinal direction.

(2) While two relatively long rectangular prisms are equal in length inthe above embodiments, they may be different in length. When the tworelatively long rectangular prisms are different from each other inlength, it is easy to discriminate between the top and bottom of thedichroic prism. Furthermore, two relatively short rectangular prisms maybe different in length.

(3) Various types of placements and structures of dichroic films in thedichroic prism are possible besides those of the above embodiments. Forexample, a green reflecting film may be provided instead of the bluereflecting film.

(4) While the present invention applies to a transmissive projectiondisplay apparatus in the above embodiments, it is also applicable to areflective projection display apparatus. The "transmissive" type meansthat optical modulation means such as a liquid crystal light valvetransmits light, and the "reflective" type means that the opticalmodulation means reflects light. In the reflective projection displayapparatus, the dichroic prism is utilized as colored-light separationmeans for separating white light into lights of three colors, red,green, and blue, and is also utilized as colored light synthesizingmeans for synthesizing again and emitting the modulated three coloredlights in the same direction. Even when the present invention applies tothe reflective projection display apparatus, it is possible to obtainalmost the same advantages as those of the transmissive projectiondisplay apparatus.

(5) While irregular reflection is given as an example of lightscattering in the above embodiments, light may be uniformly scatteredinstead of being irregularly reflected.

What is claimed is:
 1. A dichroic prism comprising:four rectangularprisms, each rectangular prism having rectangular surfaces bonded torectangular surfaces of adjacent rectangular prisms, a first surfacepart of at least one of said four rectangular prisms protruding from therectangular surfaces of other ones of said rectangular prisms, and adichroic film formed on a second surface part of said at least onerectangular prism, said second surface part not protruding from saidrectangular surfaces of said other ones of said rectangular prisms. 2.The dichroic prism according to claim 1, said first surface part beingprovided with a light diffusing layer for diffusing light.
 3. Thedichroic prism according to claim 2, said light diffusing layer being anadhesive layer.
 4. The dichroic prism according to claim 2, said lightdiffusing layer being a ground glass layer.
 5. A dichroic prismcomprising:four rectangular prisms, each rectangular prism havingrectangular surfaces bonded to rectangular surfaces of adjacentrectangular prisms, a first surface part of a first rectangular prismpair composed of two adjoining rectangular prisms of said fourrectangular prisms protruding from a rectangular surface of a secondrectangular prism pair in a longitudinal direction, and a dichroic filmformed on a second surface part of said first rectangular prism pair,said second surface part not protruding from said rectangular surfacesof said second rectangular prism pair of said rectangular prisms.
 6. Thedichroic prism according to claim 5, said two adjoining rectangularprisms in said first rectangular prism pair being in a state shiftedfrom each other in the longitudinal direction so that they form a step.7. A prism unit, comprising:a dichroic prism composed of fourrectangular prisms, each rectangular prism having rectangular surfacesbonded to rectangular surfaces of adjacent rectangular prisms, a firstsurface part of at least one of said four rectangular prisms protrudingfrom the rectangular surfaces of other ones of said rectangular prisms,and a dichroic film formed on a second surface part of said at least onerectangular prism, said second surface part not protruding from saidrectangular surfaces of said other ones of said rectangular prisms; anda prism stand for mounting said dichroic prism thereon, said prism standhaving a step that matches a step of said dichroic prism.
 8. The prismunit according to claim 7, said first surface part being provided with alight diffusing layer for diffusing light.
 9. The prism unit accordingto claim 8, said light diffusing layer being an adhesive layer.
 10. Theprism unit according to claim 8, said light diffusing layer being aground glass layer.
 11. A projector, comprising:an illumination opticalsystem for emitting illumination light; colored light separation meansfor separating the illumination light into lights of three colors; threelight modulation means for modulating the three colored lights based ona given image signal; a dichroic prism composed of four rectangularprisms, each rectangular prism having rectangular surfaces bonded torectangular surfaces of adjacent rectangular prisms, a first surfacepart of at least one of said four rectangular prisms protruding from therectangular surfaces of other ones of said rectangular prisms, and adichroic film formed on a second surface part of said at least onerectangular prism, said second surface part not protruding from saidrectangular surfaces of said other ones of said rectangular prisms; anda projection optical system for projecting the lights synthesized bysaid dichroic prism.
 12. The projector according to claim 11, said firstsurface part being provided with a light diffusing layer for diffusinglight.
 13. The projector according to claim 12, said light diffusinglayer being an adhesive layer.
 14. The projector according to claim 12,said light diffusing layer being a ground glass layer.
 15. A prism unit,comprising:a dichroic prism composed of four rectangular prisms, eachrectangular prism having rectangular surfaces bonded to rectangularsurfaces of adjacent rectangular prisms, a first surface part of a firstrectangular prism pair composed of two adjoining rectangular prisms ofsaid four rectangular prisms protruding from a rectangular surface of asecond rectangular prism pair in a longitudinal direction, and adichroic film formed on a second surface part of said first rectangularprism pair, said second surface part not protruding from saidrectangular surfaces of said second rectangular prism pair of saidrectangular prisms; and a prism stand for mounting said dichroic prismthereon, said prism stand having a step that matches a step of saiddichroic prism.
 16. The prism unit according to claim 15, said twoadjoining rectangular prisms in said first rectangular prism pair beingfixed in a state shifted from each other in the longitudinal directionso that they form the step of said dichroic prism.
 17. A projector,comprising:an illumination optical system for emitting illuminationlight; colored light separation means for separating the illuminationlight into lights of three colors; three light modulation means formodulating the three colored lights based on a given image signal; adichroic prism composed of four rectangular prisms, each rectangularprism having rectangular surfaces bonded to rectangular surfaces ofadjacent rectangular prisms, a first surface part of a first rectangularprism pair composed of two adjoining rectangular prisms of said fourrectangular prisms protruding from a rectangular surface of a secondrectangular prism pair in a longitudinal direction, and a dichroic filmformed on a second surface part of said first rectangular prism pair,said second surface part not protruding from said rectangular surfacesof said second rectangular prism pair of said rectangular prisms; and aprojection optical system for projecting the lights synthesized by saiddichroic prism.
 18. The projector according to claim 17, said twoadjoining rectangular prisms in said first rectangular prism pair beingfixed in a state shifted from each other in the longitudinal directionso that they form a step.
 19. A projection display method,comprising:emitting an illumination light; separating the illuminationlight into lights of three colors; modulating the three colored lightsbased on a given image signal; synthesizing the lights using a dichroicprism composed of four rectangular prisms, each rectangular prism havingrectangular surfaces bonded to rectangular surfaces of adjacentrectangular prisms, a first surface part of at least one of said fourrectangular prisms protruding from the rectangular surfaces of otherones of said rectangular prisms, and a dichroic film formed on a secondsurface part of said at least one rectangular prism, said second surfacepart not protruding from said rectangular surfaces of said other ones ofsaid rectangular prisms; and projecting the lights synthesized by saiddichroic prism.
 20. A projection display method, comprising:emitting anillumination light; separating the illumination light into lights ofthree colors; modulating the three colored lights based on a given imagesignal; synthesizing the lights using a dichroic prism composed of fourrectangular prisms, each rectangular prism having rectangular surfacesbonded to rectangular surfaces of adjacent rectangular prisms, a firstsurface part of a first rectangular prism pair composed of two adjoiningrectangular prisms of said four rectangular prisms protruding from arectangular surface of a second rectangular prism pair in a longitudinaldirection, and a dichroic film formed on a second surface part of saidfirst rectangular prism pair, said second surface part not protrudingfrom said rectangular surfaces of said second rectangular prism pair ofsaid rectangular prisms; and projecting the lights synthesized by saiddichroic prism.